1. Field of the Invention
The present invention relates to wireless communication systems.
2. Description of the Related Art
Communication systems, and in particular, wireless communication systems convey (i.e., transmit and/or receive) information over communication channels. In many cases, the quality of such channels varies depending on various well-known channel parameters. Certain channel parameters such as channel fading, phase jitter, frequency translation, signal to noise ratio (SNR) or carrier to interference ratio (C/I) often have a direct effect on the quality of a particular channel. The channel parameters can be measurements of specific physical quantities that occur in the channel, e.g., phase jitter, channel fading and frequency translation are representative of such physical quantities. Other channel parameters are the results of calculations of one or more physical quantities; examples of these other parameters are SNR and/or C/I. Information transmitted through the communication channels are often adversely affected by the channel parameters. In short, there is typically a direct relationship between the quality of a particular communication channel and the values of one or more of these channel parameters.
In many communication channels, particularly wireless communication channels, the quality of a particular communication channel may vary from instant to instant during transmission. The quality of a communication channel relates to the ability to transmit relatively high data rates with no or relatively few errors detected at a receive end of the channel. Thus, for a given period of time the amount of information or the rate at which information is conveyed over a particular channel may be one value and at the next period of time, the rate or amount of information can change to another value.
To address the problem of varying channel conditions, communication systems can transmit the information signals at relatively high power so that when the channel conditions are adverse, the relatively large transmit power would still allow transmitted signals to be properly detected and thus prevent errors from occurring. The use of relatively large transmit power may help reduce the error rate, but at the expensive cost of power. Therefore, many communication systems use a feedback mechanism whereby a receiver, after having received information over a communication channel, transmits channel condition information to the corresponding transmitter over a feedback channel where such channel condition information contains channel parameter values for the communication channel over which the transmitted information was received.
In a wireless communication system, the one or more communication channels through which information is transmitted from a base station to a user or mobile station is commonly referred to as the forward link. The information can be either user information or signaling information or both. User information is information exchanged between two or more users of the communication system or between a user of the communication system and another user of the communication system. The signaling information is used by the communication system to initiate, maintain and terminate communications between users or between users and equipment of the communication system. The user equipment or mobile station is able to measure and/or calculate channel parameters of the forward link from the information received from the transmitting base station. The channel parameters are referred to as channel state information (CSI).
The CSI, which represents the condition of a communication link (e.g., the forward link) at a particular instant in time, is generated by the mobile station and then transmitted over a feedback channel to the base station; that is, the mobile measures and/or calculates various channel parameters and transfers channel parameter values to a channel state matrix whose contents are transmitted over the feedback channel. The channel state matrix thus contains one or more channel parameter values, i.e., channel condition information. The base station will adjust either its transmission power, information rate or amount of information transmitted based on the channel condition information it receives from the mobile over the feedback channel. For example, the base station may reduce the transmission power because the conditions of the forward link have improved. The channel condition information is transmitted over the feedback channel in periodic fashion; that is every T seconds, channel condition information is transmitted over the feedback channel for a particular mobile station. T is a defined time period. The channel condition information can also be transmitted a periodically. Depending on the rate of transmission of the channel condition information, the amount of feedback information transmitted for each user may use a relatively large amount of communication resources such as power and bandwidth. The term bandwidth as used in this context refers to the amount of information (e.g., number of bits) that can be transmitted during a defined time period. As the throughput of wireless communication systems increases, the amount of information that is transmitted over the feedback channels increases accordingly. The throughput of a communication channel or system is the amount of information that is successfully conveyed over the channel or system during a defined time period. Successful conveyance of information refers to receiving information with no errors with a system defined acceptable amount of errors. Therefore, as communication systems increase their throughput, more resources are allocated to the feedback channels.
Certain communication systems use multiple antenna systems such as Multiple Input Multiple Output (MIMO) antenna systems in which transmitting and receiving equipment use multiple antennas to simultaneously transmit and/or receive information over communication channels. Other examples of multiple antenna systems are Multiple Input Single Output (MISO) systems and Single Input Multiple Output (SIMO) systems. MISO systems have multiple transmit antennas at one end and a single receive antenna. SIMO systems have a single transmit antenna at one end and multiple receive antennas. It should be noted that in all the aforementioned multiple antenna systems, the multiple antenna structures have multiple antennas that are capable of transmitting and/or receiving information.
Wireless communication systems with multiple antenna systems and a feedback channel arrangement have to use even more resources for the feedback channels; this is because channel condition information as seen by each one of the multiple antennas is transmitted over the feedback channel. Thus, more bandwidth is needed and correspondingly more available transmission power is needed to implement such a feedback arrangement. What is, therefore needed, is a technique that uses the bandwidth of the feedback channel more efficiently and/or a technique that uses relatively less bandwidth or a reduced bandwidth for the feedback channel of a communication system that has multiple antenna systems.